The present invention is applicable to drilling thru-holes in printed circuit board panels having at least an electrically conductive layer adjacent an electrically insulating layer, also referred to in the art as a dielectric layer. In drilling thru-holes in such printed circuit board panels, the rotary drill often smears the dielectric material onto the conductive metal portion of the hole wall. This creates many problems to which the present invention is addressed. In the description to follow, the invention is described in the context of drilling thru-holes in multi-layer printed circuit board panels having conductive metal layers separated by epoxy-glass dielectric layers; although it is recognized that the invention is applicable to the drilling of thru-holes in printed circuit board panels having other structural configurations and layers made from other materials.
Multi-layer printed circuit board panels, for example, typically include, in a cross-section, a copper outer layer, followed by an epoxy-glass resin layer, a copper inner layer, another epoxy-glass resin layer, and an outer copper layer. The number of alternating copper inner layers and epoxy-glass resin layers can vary. In some panels, the copper inner layer may not be used. In other applications, the dielectric layer may be made from a material other than epoxy-glass resin.
Multi-layer printed circuit board panels are made by a multi-stage process. Once the circuit pattern is etched on each copper inner layer, the inner layer or layers are sandwiched between the epoxy layers and the copper outer layers, and heat and pressure are then applied to cure the epoxy and bond the layers together. A large number of tiny thru-holes are then drilled through the cross-section of the multi-layer panel by a high speed drill, in preparation for forming electrical contacts between the top and bottom outer layers. Usually, the contacts are made by first plating an electrically conductive metal, such as electroless copper, to the surface of the thru-hole. The wall of the thru-hole is then further plated with electrolytic copper to complete the contact. It is not uncommon to drill thousands of thru-holes in a single printed circuit board panel. As a result, abrasion to the drill bit is a problem because of the high drilling speed and the large amount of heat generated, especially because of drilling through the metal layers and the abrasive epoxy-glass layers. For instance, the drilling tool can easily reach temperatures in the neighborhood of 500.degree. F. to 700.degree. F., and it is believed that temperatures at the tip of the drilling tool can be in excess of 1,000.degree. F. Drill speed can be sixty thousand rpm or higher.
Drilling thru-holes in the epoxy-glass and copper layers often causes the epoxy to smear over the copper inner layers. The dielectric material also can smear onto the conductive layers when drilling panels having insulating layers made from other materials. If the thru-holes are left untreated (i.e., if the smeared dielectric material is not completely removed) prior to copper plating the walls of the thru-holes, a dielectric barrier can be left between the conductive path of the copper plated thru-holes and the copper conductive layers. This interferes with the electrical connection and results in unreliable electrical contacts between the copper inner and outer layers (or between the conductive outer layers if an inner layer is not used). Accordingly, it has become a common practice to remove the epoxy smear, or other dielectric smear, before metalizing the thru-holes. (For simplicity, the dielectric smear and its associated problems are described below in relation to the known methods of treating epoxy smear.)
The prior art has disclosed a variety of techniques for removing epoxy smear, including mechanical means for abrading or physically removing the smear and chemical processes for dissolving it. Chemical processes are most commonly used. They typically comprise a multi-step process of first using highly concentrated sulfuric acid or chromic acid solutions to dissolve the epoxy resin smear. The panel then must be rinsed further to removed chemical byproducts of the epoxy smear and acid treatment. Rinsing in sulfuric acid can leave sulfonated epoxy residue on the walls of the drill holes. This can break down the integrity of the wall and require further rinsing to remove the sulfonated breakdown byproducts. If the panel is rinsed in water to remove the sulfuric acid, some of this smear can come out of solution and precipitate back on the walls of the thru-holes. Thus, various other chemicals are used for removing this precipitate which can otherwise prevent reliable electrical contacts being made between the copper layers. Neutralizers or other cleaning solutions also are commonly used in further treatment steps after acid treatment to remove epoxy smear. In addition to acid treatment, other chemical agents, such as etching solutions, are used to remove protruding glass fibers from the epoxy-glass layers. Glass fibers protruding into the thru-holes may cause misplating or result in uneven metal coverage. Further, a condition known as "pink ring" also occurs in the acid treatment of epoxy smear. Pink ring is caused by an etchback into the oxide layer at the interface between the copper layer and the dielectric layer. This leaves a void surrounding the drill hole which can interfere with contact between the hole wall and the plated copper layer.
Use of corrosive chemicals, such as concentrated sulfuric acid, chromic acid, hydrofluoric acid, and the like, has not been an acceptable solution to the problem of drilling high quality thru-holes that can be plated to make reliable contacts. These chemicals are highly concentrated (98% sulfuric acid is commonly used, for example). In addition to leaving residues that require further removal from the circuit board, the entire chemical treatment process is costly and creates disposal hazards as well. Some commonly used disposal practices are no longer permissible under new governmental regulations.
In the past, mechanical techniques used for drilling multi-layer printed circuit boards have included attempts to vary the speed of the drill to achieve entry and exit speeds that may improve the integrity of drilling through the epoxy layers. However, this approach has been unsuccessful because, as the drill heats up, the epoxy sticks to it and consequently sticks to the copper as the drill passes through the conductive layers. This also causes rough holes and leaves glass fibers protruding from the holes, and such fibers must be subsequently removed by etching or similar chemical treatment.
The present invention provides a novel approach to the process of drilling thru-holes in circuit board panels in which there is a tendency for the rotary drill to smear dielectric material on the surfaces of conductive layers penetrated by the drilled thru-hole. The invention has the unique advantage of extending drill life while at the same time essentially eliminating dielectric smear. This, in turn, reduces the need for further chemical treatment steps and eliminates treatment with harsh chemicals and the resultant toxic waste disposal problems. The invention also provides improved coverage of the metal deposited on the thru-hole surfaces when later forming the electrical contacts between the copper layers.